feat(library): lazy materialization foundation for remote tracks (§Phase1)

Adds nullable storage fields + availability column on tracks, remote
source/source_id identity on albums/artists, TrackRepository.materialize()
and get_or_create_remote() repos — groundwork for on-demand YTM library
(placeholders saved without audio, materialized in-place on first play).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/infrastructure/db/repositories/album_repository.py

@@ -18,6 +18,8 @@ def _to_entity(row: AlbumModel) -> Album:
         year=row.year,
         cover_path=row.cover_path,
         musicbrainz_id=row.musicbrainz_id,
+        source=row.source,
+        source_id=row.source_id,
         created_at=row.created_at,
         updated_at=row.updated_at,
     )
@@ -63,6 +65,58 @@ class SqlAlchemyAlbumRepository:
         await self._session.refresh(row)
         return _to_entity(row)
 
+    async def get_or_create_remote(
+        self,
+        *,
+        title: str,
+        artist_id: uuid.UUID,
+        year: int | None,
+        musicbrainz_id: str | None,
+        source: str,
+        source_id: str,
+    ) -> Album:
+        """Resolve an album by ``(source, source_id)`` first (re-browse/save
+        dedup), falling back to ``(title, artist_id)`` and gap-filling the
+        remote ids onto an existing row, else creating a new remote-bound row."""
+        row = (
+            await self._session.execute(
+                select(AlbumModel).where(
+                    AlbumModel.source == source,
+                    AlbumModel.source_id == source_id,
+                )
+            )
+        ).scalar_one_or_none()
+        if row is None:
+            row = (
+                await self._session.execute(
+                    select(AlbumModel).where(
+                        AlbumModel.title == title,
+                        AlbumModel.artist_id == artist_id,
+                    )
+                )
+            ).scalar_one_or_none()
+        if row is None:
+            row = AlbumModel(
+                title=title,
+                artist_id=artist_id,
+                year=year,
+                musicbrainz_id=musicbrainz_id,
+                source=source,
+                source_id=source_id,
+            )
+            self._session.add(row)
+        else:
+            if row.year is None and year is not None:
+                row.year = year
+            if row.musicbrainz_id is None and musicbrainz_id is not None:
+                row.musicbrainz_id = musicbrainz_id
+            if row.source is None and row.source_id is None:
+                row.source = source
+                row.source_id = source_id
+        await self._session.flush()
+        await self._session.refresh(row)
+        return _to_entity(row)
+
     async def set_cover_path(self, album_id: uuid.UUID, cover_path: str) -> None:
         row = await self._session.get(AlbumModel, album_id)
         if row is not None: